Update: Influenza Activity -- United States, 1992-93 Season

From September 27, 1992, through January 19, 1993, 344
influenza virus isolates were reported in 29 states (Figure 1). The
number of reported isolates began to increase in early December and
continued to increase during the first 2 weeks of January.

For the week ending January 16, five states (Alaska, Arkansas,
California, Missouri, and Texas) reported regional activity, and
three (New Mexico, New York, and Washington) reported widespread
activity.* The number of states reporting sporadic influenza-like
illness (ILI) increased from five states for the week ending
October 3 to 23 states for the week ending January 16. Based on
CDC's 121-city mortality reporting system, deaths associated with
pneumonia and influenza have not exceeded baseline levels.

World Health Organization collaborating laboratories in the
United States identified 98% of all isolates as influenza type B.
Although influenza type A has circulated at low levels, both
influenza type A(H1N1) and type A(H3N2) viruses have been isolated.

School outbreaks of ILI were reported from Arizona, Arkansas,
Missouri, and Washington. All of these states reported isolation of
influenza type B viruses from various sources. In Washington,
influenza type B was isolated from specimens obtained from ill
students attending schools with outbreaks.

The first outbreak this season of influenza in a nursing home
was reported from Washington. Nineteen (20%) of 97 residents became
ill during December 28, 1992-January 5, 1993; influenza type B was
isolated from three of six specimens obtained from ill residents.
As with virtually all influenza type B viruses isolated in the
United States this season, these isolates were antigenically
similar to the B/Panama/45/90-like virus included in the 1992-93
influenza vaccine. Ninety-four (97%) of the residents had received
influenza vaccine in October 1992. Two residents, both with severe
underlying diseases, died within 2 weeks of developing ILI. Most
residents, however, had relatively mild illnesses compared with
those observed in the same facility when an outbreak caused by
influenza A(H3N2) occurred during the winter of 1991-92 (CDC,
unpublished data, 1992).

Editorial Note

Editorial Note: Although most influenza viruses detected this
season have been influenza type B, health-care providers should
continue to test specimens from persons with ILI throughout the
influenza season. The proportions of different influenza virus
types or subtypes can change substantially during the season.

It is particularly important to differentiate between
influenza A or B as the cause of outbreaks of ILI in institutions
housing high-risk persons because amantadine can be used to treat
ill persons and prevent further spread of infection during
outbreaks caused by influenza type A (1). Rapid antigen-detection
testing can be performed at the site of an outbreak, and, if
present, influenza type A can be identified within 15 minutes from
a nasopharyngeal swab specimen (2). If any person in an outbreak
setting tests positive for influenza type A, it should be assumed
that influenza type A is the cause of the outbreak. Results of
rapid antigen-detection tests should be confirmed by virus
isolation. However, when a decision has been made to use
amantadine, initiation of amantadine prophylaxis or treatment
should not be delayed pending confirmation of virus type.
Guidelines for the use of amantadine to control influenza type A in
chronic-care facilities have been published (1).

With the increase in influenza activity, it is important that
children and teenagers avoid the use of aspirin and
aspirin-containing products because of the increased risk of
developing Reye syndrome when aspirin is taken during an ILI (3).

*Levels of activity are: 1) sporadic -- sporadically occurring
influenza-like illness (ILI) or culture-confirmed influenza, with
no outbreaks detected; 2) regional -- outbreaks of ILI or
culture-confirmed influenza in counties having a combined
population of less than 50% of the state's total population; and 3)
widespread -- outbreaks of ILI or culture-confirmed influenza in
counties having a combined population of 50% or more of the state's
total population.

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